Field of the Invention
The invention concerns a method and apparatus for combined dual-energy mammography and tomosynthesis imaging of a predetermined volume of an object under examination while a contrast agent is flowing through the volume.
Description of the Prior Art
In x-ray mammography, tomosynthesis scans of the breast and their three-dimensional reconstructions as tomograms are state of the art as a diagnostic method, while two-dimensional mammography images (mammograms) are used primarily for monitoring (screening) procedures and as images for comparison with images previously recorded. Digital tomosynthesis is understood as meaning a combination of digital image capture and image processing, with minimal movement of the x-ray source. Tomosynthesis has similarities to computed tomography (CT), but is regarded as a separate modality. While in the case of computed tomography, projection images are created during a full 360° rotation of the x-ray source around the object under examination, in the case of tomosynthesis the x-ray source rotates only through a small angle, for example 40°, with only a small number of projection images being created (typically between 7 and 60). The use of highly resolving detectors allows a very high resolution in planes perpendicular to the axis referred to as the Z axis (the axis in the direction of the 0° tomosynthesis angle or the perpendicular direction from the x-ray source to the detector or CC alignment (cranial-caudal=from head to foot)), even if the resolution in the direction of the Z axis is less. In comparison with mammography, tomosynthesis operates with a lower radiation dose per projection.
Also known is the mode referred to as the combined 2D/3D mode, in which two-dimensional mammographic images are also recorded within a tomosynthesis scan. The typical procedure here is that, with an inserted anti-diffusion grid in the central (0° CC) direction of projection (for CC=cranial caudal and MLO=medio lateral oblique), after the recording of the pre-shot, which is used for calculating the mAs required for the projections, a main shot of x-radiation is triggered, creating the mammographic image. Subsequently, the anti-diffusion grid is withdrawn and the tomosynthesis scan is started. By this combined mode, both a two-dimensional mammographic image (mammogram) and a 3D tomogram of the breast in the same position and under identical compression are created.
Contrast-enhanced dual-energy mammography (CEDEM) is a relatively new diagnostic method. In this method, with a contrast agent flowing through the patient, two-dimensional low-energy images and two-dimensional high-energy images are respectively recorded and subsequently subtracted from one another, in order to make the concentration of the contrast agent visible. The application of CEDEM to 3D imaging is also known (contrast-enhanced dual-energy tomosynthesis, CEDET). Virtual 2D mammographic images calculated from a tomosynthesis scan are also known, but are not regarded as fully equivalent to a recorded 2D mammographic image. DE 10 2010 041 920 A1 discloses a method in which first a low-energy image, then a high-energy image and a high-energy tomosynthesis scan are created and the low-energy image is subtracted from the high-energy images.
Images recorded without a grid (that is to say an image recorded without an anti-diffusion grid for filtering out undesired diffused radiation) for contrast-enhanced mammography are known. In these cases, the diffused radiation correction is performed by software (for example Siemens Inspiration PRIME=Progressive Reconstruction Intelligently Minimizing Exposure). A disadvantage of this solution is that it is not associated with a tomosynthesis scan (from different directions of projection) and always leads to different breast positioning, which makes a diagnosis more difficult (for example due to poor positioning, nipple retraction, cutting off of the tissue near the breast wall, etc.) or alternatively leads to much longer breast compression for the patient.